AUTUMN AND WINTER DAILY PRECIPITATION AREAS IN WALES, 1982-1983 TO 1986-1987

Daily precipitation data from 121 sites for five autumn-winter seasons (September to January, 1982-1983 to 1986-1987) have been analysed to determine discrete precipitation areas within Wales, using S-mode principal components (PCA) and cluster analyses. Using unrotated PCA, much of the variation in daily precipitation is explained by the first five PCs, representing an accumulated variance of 75.7 per cent. The first four PCs appear to relate respectively to: altitude (60.8 per cent of the total variance explained), an exposure effect relative to north or north-east winds (6.0 per cent), a 'westerly exposure' effect (4.2 per cent) and an 'easterly exposure' effect (2.9 per cent). The fifth PC, with high scores only over Ynys Mon and Gwent, could not be attributed to any particular influence. Oblique rotation produced a better simple structure, enhanced the apparent importance of exposure to wind from the main compass directions, but lessened that of altitude. Three methods of clustering PCs were subsequently tried on the Oblimin solution: centroid, median and Ward. The precipitation areas produced by each were broadly similar, and the boundaries between them often coincided with clear topographic units, influenced by exposure and shelter effects. The Ward technique yielded the areas best defined in terms of topography and exposure, and best satisfied criteria determined by the nature of the data themselves.